Country borders are not rendered about openstreetmap-carto-ajt

[SomeoneElseOSM]

(and finally for completeness) https://wiki.openstreetmap.org/wiki/User:SomeoneElse/Ubuntu_1804_tileserver_load and https://switch2osm.org/manually-building-a-tile-server-18-04-lts/ are fairly similar, but use different map styles. Both evolved from the same root (the "standard" style in use on openstreetmap.org a few years ago), but have different goals and different limitations.

If you'd like to have a map style that creates tiles that look exactly the same as you see on openstreetmap.org then you can just follow the switch2osm.org guide and that's what you'll get. If you're creating a background map for something else then you'll probably want to make some features more or less visible, and modify the map style you use in some way - you can do that with either style (and with many others).

[SomeoneElseOSM]

Everything works fine but the country boundaries are not rendered

Yes - that's by design. This particular map style doesn't show administrative boundaries by default as part of the map tiles, although it's possible to add them on as an overlay afterwards.

The actual bit that removes the admin boundaries is here: https://github.com/SomeoneElseOSM/SomeoneElse-style/blob/master/style.lua#L891 .

Removing admin boundaries means that purple is available as a colour for other features - different shades of it are used for long distance walking, cycling and horse-riding paths.

If you want to show admin boundaries as an overlay, have a look at https://www.openstreetmap.org/user/SomeoneElse/diary/47007 which explains how I did it at https://map.atownsend.org.uk/maps/map/map.html . If you'd like to have admin boundaries rendered as part of the map tiles, then comment out the lines in style.lua that remove them.

There are advantages and disadvantages of both methods. An advantage of having boundaries in a separate layer is that you can choose whether or not to display them; a disadvantage is that label collisions between tile layers can occur (see examples noted in this diary comment: https://www.openstreetmap.org/user/SomeoneElse/diary/47007#comment43366 ).